Description
The invention relates to a metal closing disk and to a process for producing this closing disk or for improving the seal between the surface of a metal disk and an insulating layer applied to it.
A metal closing disk of the relevant type, also intended particularly for can-type capacitors, is known from DE No. 2,718,228 A1. However, the main feature of this known closing disk for can-type capacitors is that the closing disk itself is designed in construction terms so that as a mass-produced article it can be provided automatically with an insulating layer, above all in the region of the disk edge.
In this known design of the closing disk, the aspect of achieving as good and constant electrical data as possible in an electrolytic can-type capacitor provided with this closing disk still requires improvement. It has been shown, in particular, that when, for example, an elastomer compound is injection-molded round the disk edge it is still always possible for electrolytic substances, here especially liquid electrolytes, to penetrate between the elastomer layer and the underside or elevation surface side of the closing disk. This process occurs when only an outer radial edge region of the elevation surface side of the closing disk is covered by the insulating layer, but also when the insulating layer extends completely up to the bar-shaped metal elevation which in the known closing disk is designed as a centering or winding spindle.
The reason for the change in electrical characteristics, here especially the capacitance of the appropriate finished electrolytic can-type capacitor, is that, when a least the surfaces of the closing disk including the metal elevation, which point into the capacitor can, are preformed, postforming caused by the electrolyte takes place when the capacitor is put into operation. It is assumed here, of course, that the closing disk forms the positive pole of the capacitor, which is electrically insulated from the can housing, which serves as a negative pole, by means of the insulation of the closing disk in the edge region.
Whereas the postforming of the surface of the known closing disk, this surface being as it were exposed relative to the can interior of the capacitor, can take place so to speak by the application of voltage or when the capacitor is put into operation, the elastomer layer, for example vulcanized on the elevation surface side, prevents postforming taking place parallel to the other regions. The surface boundary layer, pointing into the can housing, of the closing disk, including the metal bar-shaped elevation, is consequently partially postformed and partially only preformed. The resulting different forming layers represent, when the capacitor is in operation, potential differences, slight though they may be, which cause residual currents and tracking currents as fault currents. This problem becomes even more serious when the inner surface of the closing disk is not preformed at all and when gradual postforming takes place on the surface not covered by the elastomer. The capacitances of electrolytic can-type capacitors are greatly influenced by these fault sources, so that sometimes not even the tolerance values are maintained or relatively large tolerance ranges have to be specified for the capacitance. However, in both cases, there are serious disadvantages, so that efforts are being made to eliminate these defects.
European Patent Application EP No. 0,065,021 A1 also discloses a metal closing disk which is designed as a composite cover disk, the main aim of this application being to allow several electrical ducts to be guided, electrically insulated, through the closing disk and, furthermore, to integrate an excess-pressure safety valve in this closing disk. As can be seen particularly in the Figures, the metal closing disk is likewise designed with an elastomer layer surrounding its edge and extending onto the underside. At a slight distance from this elastomer layer and possibly even adjoining it, a plastic layer is provided on the underside. In a similar way to the known closing disk mentioned above, this adjoining region between the plastic and elastomer layers on the underside allows the electrolyte to penetrate into this boundary layer during operation and gradual forming of at least the underside of the cover disk to occur. As a result, even in this cover disk, the disadvantages described above, especially that of a change in capacitance, can arise.
The object of the invention is, therefore, to design a closing disk of the relevant type, which it must be possible to produce economically as a mass-produced article, in such a way that a reliable seal against substances in the can interior, especially against an electrolyte, is achieved between the insulating layer and the underside of the closing disk in the region of the bar-shaped elevation, and this will result in additional fixing of the insulating layer and must be possible essentially without further devices even when the closing disk is in one piece with the bar-shaped elevation, the object also being to provide an appropriate process for this.
As regards the process, the object is achieved by means of the features of the characterizing clause of claim 11.